RCC (Ringing choke convertor) is widely used as LED driver circuit and charger due to low cost. However it is not widely used in the applications which require high performance. It's very hard for RCC circuit to achieve high Power factor, low THD, good line regulation at the same time.
FIG. 1 shows an RCC-based converter circuit which is a buck converter from CN204104165U and WO2015/181665A1. The energy storage inductor is L2 and the power switch is Q1. Self-oscillation is realized by auxiliary winding L3 magnetically coupled to the energy storage inductor L2 and connected to the base of the power switch Q1. Resistor R16 and R17 sense the power current through the power switch Q1 and feed back this information to a switch set such as the Darlington bridge Q2 and Q3, in order to regulate the base current of the power switch Q1. More specifically, the Darlington bridge Q2 and Q3 operates in on/off state to turn off/on the power switch Q1.